1. Field of the Invention
The present invention relates to an organic electroluminescent element that converts electric energy to light and luminesce.
2. Description of the Related Art
An organic electroluminescent element is made of an organic compound layer that includes at least a luminescent layer and a pair of electrodes that sandwich the organic compound layer. When an electric field is applied between both electrodes, electrons are injected from a cathode and holes are injected from an anode. The electrons and holes recombine in a luminescent layer to luminesce.
One important problem in an organic electroluminescent element is a problem of development of a blue phosphorescent organic electroluminescent element (hereinafter, referred to as “blue phosphorescent luminescent element” for convenience). When a blue phosphorescent luminescent element is designed, it is important to achieve firstly high luminous efficiency and secondly high driving durability.
As a blue phosphorescent luminescent element that takes the problem into consideration, for instance, Japanese Patent Application Laid-Open (JP-A) No. 2004-6287 discloses a blue phosphorescent luminescent element in which differences of energy levels of LUMO and energy levels of HOMO in between a hole block layer and a luminescent layer and relationship between a band gap and a molecular weight of a host compound are taken into consideration. However, the blue phosphorescent luminescent element disclosed in the literature is not sufficiently high in the luminous efficiency and the driving durability.
Furthermore, in a luminescent layer of a blue phosphorescent luminescent element, a blue phosphorescent luminescent material and a host material are included. The blue phosphorescent luminescent material normally has the lowest excited triplet energy (T1) (hereinafter, referred to as “T1 energy” for convenience) of 272 kJ/mol (65 kcal/mol) or more. Accordingly, in order to achieve high luminous efficiency, a host material having a T1 energy of 272 kJ/mol (65 kcal/mol) or more is necessary. However, the host material having a T1 energy of 272 kJ/mol (65 kcal/mol) or more is difficult to inject with electric charges (holes or electrons), as a result, there is a problem in that the driving durability is poor.
Thus, at present, a blue phosphorescent luminescent element that combines high luminous efficiency and high driving durability is not yet provided.